Add ginger garlic paste and cook another minute. Mix in the ground spice mix and stir for about 2 minutes. Add a splash of oil if it looks too dry. Then add the diced tomatoes, yogurt, and, if using, cashew nut paste. Reduce heat to low, cover, and cook 2-3 minutes. Remove cover and add shrimp to cook a few minutes for them to turn pink and the sauce to dry a bit. Transfer to a serving bowl, sprinkle with cilantro, and serve hot with rice.